Electromigration is the transport of material caused by the gradual movement of ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in microelectronics and related structures. Also, as the structure size in integrated circuits (ICs) decreases, the practical significance of this effect increases.
There are two failure modes for electromigration for via interconnects and wiring structures in dual damascene copper structures: (i) via depletion; and (ii) line depletion. Via depletion occurs when electrons flow from a wiring line below into the via interconnect above. For this mode, slowing down the copper diffusion along the copper cap interface above the via interconnect is important. On the other hand, line depletion occurs when electrons flow from the via interconnect down to the wiring line below. For this mode, slowing down the copper diffusion under the via interconnect is important. Taking these different modes into consideration, a requirement for a specific wire may be driven by either performance or electromigration, but the resistance of all of the wiring will be determined by the electromigration requirements of a worst case electromigration.